1. Field of the Invention
The present invention relates to an ion chromatograph for separation and analysis of inorganic ions or organic ions contained in a sample solution and a suppressor for suppressing the background electrical conductivity of an eluate discharged from a separation column of such an ion chromatograph.
2. Description of the Related Art
In an ion chromatograph, a sample is introduced into a separation column to separate it into component ions, and then the component ions are detected by measuring the electrical conductivity of an eluate discharged from the separation column in an electrical conductivity measurement cell. In order to achieve high-sensitive measurement, a suppressor is provided between the separation column and the detector. The suppressor removes nontarget ions contained in an eluate discharged from the separation column to reduce the electrical conductivity of the eluate.
As such a suppressor, one using an ion-exchange membrane is conventionally used. An example of a conventional suppressor is shown in FIGS. 7A and 7B. FIG. 7B is a sectional view of the conventional suppressor shown in FIG. 7A taken along the X-X line. A channel 104 through which an eluate discharged from a separation column flows, and a channel 106 through which a regenerant for regenerating ionic functional groups of an ion-exchange membrane 102 flows are arranged so as to be opposed to each other through the ion-exchange membrane 102. Base bodies 108 and 110 are arranged so that the channels 104 and 106 are opposed to each other with the ion-exchange membrane 102 being interposed between them.
The channels 104 and 106 are merely hollow channels through which liquid flows. The ion-exchange membrane 102 has low stiffness, and therefore, when the pressure in the channel 104 and the pressure in the channel 106 are changed due to a change in the back pressure of the suppressor, and thus, a large difference is caused between the pressure in the channel 104 and the pressure in the channel 106, the ion-exchange membrane 102 is displaced toward one of the channels due to pressure exerted on the ion-exchange membrane 102. The displacement of the ion-exchange membrane 102 causes changes in the volumes of the channels 104 and 106, and as a result, the amount of nontarget ions removed from an eluate fluctuates and therefore the base line of a chromatogram becomes unstable.
In order to prevent such displacement of the ion-exchange membrane, a suppressor obtained by filling the channels 104 and 106 with an ion-exchange resin as a filler (see JP-A-1-169353) and a suppressor obtained by filling the channels 104 and 106 with a cross-linked material (see JP-A-61-172057) have been proposed.
In a method for filling channels with a filler, such as the proposed suppressor, pressure losses in the channels are increased by the filler and therefore, liquid feed pressures need to be increased. This, however, increases the load on the ion-exchange membrane, which may cause disadvantages, such as leakage of liquid from parts fixing the ion-exchange membrane.
Further, in a case where the filler is an ion-exchange resin, there is a case where a difference in performance is caused among individual suppressors due to variations in the characteristics of the ion-exchange resin or the activity of the ion-exchange resin is changed due to repeated use so that changes in ion chromatogram occur with time.